1. Field of the Invention
The present invention relates to an ultrasonic diagnostic apparatus that extracts microstructures from internal organs on the basis of echo signals from tissue, and a method of controlling the same.
2. Description of the Related Art
Ultrasonic diagnosis makes it possible to display how the heart beats or the embryo moves in real time by a simple operation of tapping an ultrasonic probe on the body surface, and allows repetitive examination because it is highly safe. In addition, the size of an ultrasonic diagnostic system is smaller than other diagnostic apparatuses, such as X-ray, CT, and MRI apparatuses, and the apparatus can be moved to a bed side to allow easy examination. Ultrasonic diagnostic apparatuses used for the ultrasonic diagnosis vary depending on the types of functions which they have. For example, an ultrasonic diagnostic apparatus having a size that allows an operator to carry it with his/her one hand has been developed, and ultrasonic diagnosis is free from the influence of radiation exposure such as X-ray exposure. Therefore, the ultrasonic diagnostic apparatus can be used in obstetrical treatment, treatment at home, and the like.
This type of ultrasonic diagnosis includes breast cancer early diagnosis. It has been known that, in many cases, micro calcification occurs in breast tissue as a symptom of breast cancer. One or a few calcified lesions are scattered in local portions. Since lime is harder than living tissue, it reflects ultrasonic waves well. Therefore, such a calcified lesion is expected to exhibit high brightness on an image. However, it is difficult to extract such a lesion from an image by visual recognition even if it has a size of about several hundreds of microns.
In some cases, interference fringes called speckle patterns due to random interference between ultrasonic waves are generated on an ultrasonic image. On the other hand, this speckle pattern is used to diagnose, for example, hapatocirrhosis. For example, this speckle pattern is very similar to a microstructure such as a micro calcified substance which tends to be overlooked in, for example, the above breast cancer diagnosis. In some cases, the speckle pattern becomes misleading image information. For this reason, in the above breast cancer diagnosis which requires no speckle pattern, in order to remove the speckle pattern, for example, the following processing is performed: spatial compounding, CFAR (contrast false alarm rate) processing, and a similarity filtering process. The spatial compounding is to overlap signals that are transmitted or received in different directions and smooth the speckle pattern. The CFAR processing is to subtract a target pixel from the average brightness of adjacent pixels of the target pixel and extract a high-brightness portion on the basis of the difference. The similarity filtering process is to remove the speckle pattern using its statistical property. These techniques are disclosed in, for example, Japanese Unexamined Patent Application Publication Nos. 61-189476, 2001-238884, 2003-61964, and 2004-321582. Although not included in the ultrasonic diagnosis field, in addition to these techniques for removing the speckle pattern, various attempts to automatically recognize micro calcification have been reported mainly as applications of X-ray diagnostic images (for example, Japanese Patent No. 3596792).
Further, an MIP process has been proposed as another technique for extracting a microstructure, such as micro calcified substance. The MIP process projects a representative value, which is the maximum brightness of a plurality of image frames, onto one frame. The MIP process is mainly used to display volume data as a two-dimensional image during three-dimensional image processing. Ideally, it is possible to superpose information of a plurality of frames on one frame to obtain an image having a large amount of information. Further, it is also possible to change image quality adjusting parameters provided in an ultrasonic diagnostic apparatus according to the related art to reduce the speckle pattern. Furthermore, for example, when a dynamic range is narrowed, only specific signals in a narrow range are displayed. When optimum setting, such as the setting of low gain, is performed, it is possible to display only microstructures having relatively high signal intensity without displaying the speckle pattern having relatively low signal intensity.
However, for example, the following problems arise in the above conventional techniques for extracting microstructures.
For example, the mammary gland, which is a diagnosis target, is an internal organ that has a complicated structure of breast duct and the like and is not homogeneous. Therefore, when the filtering process according to the related art is performed, both a micro calcified substance and the mammary gland are extracted (as structures), and the operator is difficult to clearly discriminate them.
Further, for example, a breast duct is significantly larger than a micro calcified substance, it is expected that the operator will discriminate the breast duct from other substances by eyes after the filtering process. However, the inventors' studies proved that the filtering process was insufficient to clearly discriminate the breast duct from other substances. In particular, when a portion of the mammary gland structure remains, a filter image seems to be a dot, which is similar to the image of a micro calcified substance.